An apparatus of the abovementioned type, primarily for displaying two-dimensional images, is already known through our Swedish Patent Application 9101778-0, corresponding to allowed U.S. patent application Ser. No. 07/897,471, now U.S. Pat. No. 5,327,153. A three-dimensional display is produced in this case by supplying to a two-eyed observer a separate part-image for each eye, so-called stereo image pair. However, three-dimensional display according to the abovementioned patent application has certain limitations. Due to the fact that the image is built up of only two part-images, there is very limited information about the three-dimensional image seen from one direction, and the apparatus requires that the observer have two eyes.
Optical imaging in the traditional sense can be considered as a projection of a three-dimensional aerial image on an image plane, for example a ground glass, a film or the retina of the eye. If there is sufficient depth of field within the object space of interest when it is imaged on a plane, this is not felt to be a serious restriction. In this way, however, only a subset of the information content of the complete three-dimensional image volume is made use of. As a rule, each image plane is accessible to an observer by refocusing, whereby only one plane at a time can be registered. This is especially pronounced in optical systems with high light-gathering power and therefore little depth of field such as, for example, a microscope or electrooptical instrument with some image-converting component and a display screen, for example image intensifier, TV camera, IR camera and so forth.
Traditional optical imaging is passive, that is to lacking illumination of the target with some time tagging. It is thus not possible to measure the distance to different objects with the aid of time measurement. One possibility is to measure the angle to the target from two positions, for example within one lens aperture. The image of the target can be set to be sharp by shifting a lens. The greater the range of angles received from the object, that is the larger the lens in relation to the focal length, the less the uncertainty in depth and therefore the less the "depth of focus" obtained. Distant parts are "pressed together" which is the result of the lens formula. According to the lens formula, it holds true that a.multidot.b=f.sup.2, where a is the object distance to the front focal point, b is the image distance from the rear focal point and f is the focal length. An example of passive imaging with possible improvements according to this method is a microscope with a large depth of field and good reading possibilities on a three-dimensional screen.
Active sensors of the radar echo type with some form of time tagging of the transmitted signal provide a possibility for linear scales in all three dimensions, distance, angles in azimuth and elevation. Examples of this system are radar, laser radar, sonar and so-called "gated viewing". As another example of a use of active sensors, shadow projection methods with linear image synthesis, for example X-ray tomographs, magnetic cameras and so forth can be mentioned, which are mainly used in medical diagnostics.
In the text which follows, actual bodies in the environment are called "objects". Images of these, for example generated by light, IR radiation, radar, X-ray, acoustic waves and so forth, are called "primary images". A "primary image" can also be completely synthetic. A "display" which makes a "primary image" visible to the observer is then produced with the aid of two separate "display elements".